1. Field of the Invention
This invention relates to a method for optimized post-combustion of CO and unburned hydrocarbons in combustion processes where low levels of NOx emissions are to be achieved.
2. Related Art
High-temperature, natural gas-fired furnaces, especially those fired with preheated air, produce significant quantities of nitrogen oxides (NOx) per unit of material processed. At the same time, regulations on emissions from industrial furnaces are becoming increasingly more stringent, especially in areas such as California.
Consequently, there has been a demand for improved combustion technologies allowing reduction of NOx formation. Different solutions have been developed, usually based on the principle of either staged or diluted combustion. However, the operating conditions that favor the reduction of NOx emissions typically affect the combustion process itself such that combustion can become incomplete, thereby generating carbon monoxide (CO) and unburned hydrocarbons (HC). For this reason, and in order to achieve optimum emission performances, some of the low NOx technologies have had to be coupled with some sort of post-combustion system in order to remove CO and unburned HC from the flue gas before being exhausted into the atmosphere.
The present invention relates to industrial combustion processes, including high-temperature furnaces, industrial boilers, and utility boilers, facing stringent NOx regulations. Because nitrogen oxides (primarily NO and NO2, hereafter NOx) have been identified as a major cause of air pollution as well as a significant health hazard in ambient air, they have been defined as a criteria pollutant by the Clean Air Act Amendment (CAAA), which has established environmental limits in determined locations.
To comply with these regulations, many U.S. combustion process operators have had to implement NOx control technologies in the past few years. This trend will most likely propagate in other areas and become even more pronounced.
Widely implemented low NOx technologies included combustion techniques, allowing to significantly prevent the formation of NOx inside the combustion chamber, in contrast to post-treatment techniques (such as Selective Catalytic Reduction), where NOx is removed from flue gases through chemical reactions. Among these combustion-based NOx control technologies, many different techniques have been proposed and optimized, based on the following concepts: 1) reduction of the temperature in the combustion zone to limit NOx formation mechanism, 2) decrease of the oxygen concentration available for NOx formation in the high temperature zones, and/or 3) creation of conditions under which NOx can be reduced to molecular nitrogen by reacting with hydrocarbon fragments.
One example of this general type of technology is low excess air, or reducing the available oxygen to the point which is just sufficient to oxidize the fuel but not so much as to cause emissions such as NOx, and CO (i.e. stoichiometric balance). Another example is staged combustion, or staging combustion by arranging the inlets of fuel or air to achieve off-stoichiometric firing conditions in the different zones of combustion. Still another example is flue gas recirculation (FGR), or recirculating the flue gas to the combustion zone as a diluent to reduce flame temperature and oxygen concentration. Another example is oscillating combustion, or oscillating the flow of fuel in order to create fuel-rich and fuel-lean combustion zones, and operating only under off-stoichiometric conditions. A final example is gas reburning, or introducing fuel gas to bum in the post combustion zone, generating hydrocarbon fragments which reduce the NOx formed in the main combustion zone to molecular nitrogen.
Advantageous as the foregoing examples can be, they each suffer some drawbacks. When applied to minimize NOx production, these examples can affect the mixing of the reactants, generating instability in the combustion process, and eventually causing incomplete combustion. The result is the unwanted exhaust of CO and unburned hydrocarbons from the combustion system. This can be explained by the fact that CO and unburned hydrocarbon formation is dependent on the same three basic factors that influence NOx emissions: temperature, oxygen concentration and residence time at elevated temperatures. Unfortunately, each of these must be controlled in the opposite direction from that of NOx reduction: if all three factors are decreased, NOx production can be dramatically reduced but CO production is enhanced, and vice versa.
As CO emissions can rarely be sacrificed for reduced NOx because low emission systems must keep both pollutants at a minimum, it is then necessary to implement a CO removal system downstream of the combustion region. Again two different approaches are basically available in the prior art: a first one, based on catalytic oxidation of CO in the flue gas exhaust section, at reduced temperature, and a second one, based on post-combustion of CO with an oxidant, inside or in the vicinity of the combustion chamber.
A well known catalytic method is the Non-Selective Catalytic Reduction (NSCR), used in rich burn engines for simultaneous reduction of NOx, CO and volatile organic compounds. With this method, the engine is tuned to run richer so that there is a concurrent decrease in NOx and increase in reducing agents (CO and HC). Then downstream of the engine, in the presence of a catalyst and at reduced temperature, NOx react with CO, HCs or H2 to produce nitrogen, carbon dioxide and water. If this method allows a combined reduction of the different pollutants, it can however be quite expensive, with the need for a tight process control system and a costly catalytic reactor that has to be replaced periodically. A similar system for gas-fired heating units is also presented in German Patent DE 4006735, issued to Ragert.
Thus, a problem associated with high-temperature CO removal methods that precede the present invention is that they do not provide a post-combustion system including at least one, and preferably a combination, of a flue gas recirculation zone, a flue gas mixing zone and an oxidant injection zone.
Still another problem associated with high-temperature CO removal methods that precede the present invention is that they do not provide a method that sufficiently increases the residence time of combustion products inside the combustion chamber, especially in regions where temperature is low enough to prevent the formation of NOx.
Another problem associated with high-temperature CO removal methods that precede the present invention is that they do not provide enhancement of the mixing of combustion products inside the combustion chamber in order to favor the completion of the combustion.
An even further problem associated with high-temperature CO removal methods that precede the present invention is that they do not provide optimized post-combustion oxidant injection devices that distribute as evenly as possible this oxidant into the post-combustion zone.
For the foregoing reasons, there has been defined a long felt and unsolved need for a method for efficiently and cost-effectively removing CO and unburned HC from the flue gas of low NOx combustion processes without re-creating substantial NOx emissions.